How to choose a scanner?

I am currently looking into purchasing a new scanner. Last time I bought one was probably about 8 years ago, and it doesnt even work with my computer since I upgraded to Windows XP a few years ago.

I am looking at the Epson Perfection series of scanners, because I have heard they give you very good image quality for a low price. There are a number of different models available though, and I'm not really sure what the difference between any of them is, except the price! I know that the more expensive models allow scanning at a higher DPI than the cheaper ones, but even the cheapest model seems to have an extremely high optical DPI (at least compared to my old one =P ). I have also heard that when scanning photographs, you typically shouldnt need to scan at any higher than 600dpi, unless you are enlarging the image. Is there any truth to this, and does that mean that I would essentially be wasting my money by buying a scanner with more DPI than I really need?

The one that I have my eye on is the Epson Perfection 3170. There is also the Epson Perfection 1670, which is quite a lot cheaper, and only has a little less DPI. And then, there are also a number of higher model scanners that I could consider.

I guess my question is just basically, what am I really getting when I pay more for a scanner?

One time you may use the higher resolutions is if you are scanning negatives or transparencies. My understand is most of the improved specs on the higher models are in this area, rather than improving scans of prints. But this is just my assumption.

I have the Perfection 2480 (Not sure if that's what it's called in America though). I'm fairly happy with it - though I have been a bit dissappointed at the level of distortion across the platten, which is only apparent when you are trying to calibrate two rotated scans together perfectly (in an attempt to remove paper texture)